Apparatus for improved low pressure inductively coupled high density plasma reactor

ABSTRACT

A plasma reactor comprises an electromagnetic energy source coupled to a radiator through first and second variable impedance networks. The plasma reactor includes a chamber having a dielectric window that is proximate to the radiator. A shield is positioned between the radiator and the dielectric window. The shield substantially covers a surface of the radiator near the dielectric window. A portion of the radiator that is not covered by the shield is proximate to a conductive wall of the chamber. Plasma reactor operation includes the following steps. A plasma is ignited in a chamber with substantially capacitive electric energy coupled from the radiator. A variable impedance network is tuned so that the capacitive electric energy coupled into the chamber is diminished. The plasma is then powered with substantially magnetic energy.

This application is a Divisional of U.S. application Ser. No.10/263,624, filed Oct. 3, 2002, which is a Continuation of U.S.application Ser. No. 09/031,400, filed Feb. 26, 1998, now U.S. Pat. No.6,516,742, which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuitfabrication, and more specifically to integrated circuit fabricationwith plasma reactors.

BACKGROUND OF THE INVENTION

Plasma reactors are commonly used for semiconductor processing. Asdisclosed in U.S. Pat. No. 4,918,031 to Flamm et al. (hereinafter “theFlamm Patent”), herein incorporated by reference, plasma reactors areused for material formation and removal on wafers. Material formation isaccomplished by plasma enhanced chemical vapor deposition (PECVD). WithPECVD, a plasma and precursor gases are combined in a reactor to form amaterial on a wafer. The precursor gases may be, for example, silane andammonia. The plasma provides the energy to breakdown the constituentelements, such as silicon and nitrogen, of the precursor gases.Constituent elements that have high affinity for one another combine toform the desired material, for example silicon nitride, on the wafer.

Alternatively, plasma reactors are used to remove material from thewafer by etching. The plasma reactor can be used during differentprocessing steps to remove different materials, including oxides,aluminum, polysilicon and silicides, from a wafer.

If used for etching a wafer, the plasma reactor will contain a reactivegas, such as a fluorocarbon or chlorine at a low pressure. The type ofgas used in the plasma reactor depends on the corresponding material tobe removed from the wafer. It is preferable to etch specific materialson the wafer with plasmas of certain gases because the plasmasselectively etch the specific materials at relatively high rates.

Construction of a plasma reactor will now be described. The plasma,gases, and wafer are contained in a chamber of the reactor. The chamberis substantially formed by metal, such as aluminum, that is electricallygrounded. Portions of the chamber interior may be covered with aninsulating liner, made from quartz or alumina for example. The chamberhas at least one open surface that is sealed by a dielectric window. Thedielectric window may be formed from quartz, alumina, silicon nitride oraluminum nitride.

Outside the chamber, a radiator, such as a coil, is placed proximate tothe dielectric window. The coil may be planar or cylindrical. Forexample, the TCP series of plasma reactors 120 made by Lam Research,Inc. (Fremont, Calif.) uses a planar coil 122, as illustrated in FIG.1A. However, the HDP series of plasma reactors 130 manufactured byApplied Materials, Inc. (Santa Clara, Calif.) uses a cylindrical coil132, as shown in FIG. 1B. Different coil designs are also described inthe Flamm Patent and U.S. Pat. No. 5,234,529 to Johnson (hereinafter“the Johnson Patent”) which is herein incorporated by reference. Oneterminal of each coil is coupled to an electromagnetic energy source.The electromagnetic energy source is typically operated at radiofrequencies. The coil and dielectric window permit the transmission ofelectric and magnetic energy from the electromagnetic energy source intothe chamber. The energy is used to ignite, or strike, and then power theplasma created from a gas. Typically, the coil is designed to makeelectrons in the plasma travel in a toroidal pattern which has a radiusabout equal to the radius of the wafer. The coil is also designed toexcite the plasma so that it will uniformly affect, such as etch, thewafer.

It is desirable to increase the amount of energy transferred from theelectromagnetic energy source to the plasma. Hence, avariable-impedance-matching network is inserted between theelectromagnetic energy source and the coil in order to achieverepeatable, controlled delivery of energy to the plasma. Thevariable-impedance-matching network is adjusted during plasma reactoroperation to enhance the energy transfer between the electromagneticenergy source and the plasma.

To operate the reactor, the plasma must be first ignited and thenpowered. The plasma is created from a gas by igniting the gas withenergy radiated from the coil. Specifically, the plasma is ignited byaccelerating free electrons in the chamber into molecules of the gas. Asa result, the gas molecules are ionized. If a sufficient number ofmolecules are ionized, an avalanche effect is created and the plasma isignited.

Free electrons can be accelerated with electric and magnetic fields.However, practically, only capacitive electric energy can be used toignite the plasma. The force exerted on a free electron by thecapacitive electric energy in the chamber prior to ignition is muchlarger than the force exerted by the magnetic energy. The force from themagnetic energy is relatively small because its strength is proportionalto the velocity of the free electrons, which is also small before theplasma is ignited.

Thus, the plasma is preferably ignited by an electric field between thecoil and a conducting wall of the chamber. However, once the plasma isignited, the capacitive electric energy can have a detrimental effect onthe reactor. The capacitive electric energy causes insulating materialfrom the insulating liner to be sputtered onto the dielectric window. Asa result, the insulating liner is depleted during the course of plasmaoperation. The insulating material sputtered onto the window may falloff and contaminate the wafer. Also, because the voltage is not uniformacross the coil, the amplitude of the capacitive electric energy in thechamber and incident on the wafer is also not uniform. Because someprocess parameters are dependent upon the amplitude of the capacitiveelectric energy incident across the wafer, undesirable variations ofprocess parameters on the wafer may occur during wafer processing in theplasma reactor. As a result, the structure and electrical performance ofthe integrated circuits formed on the wafer may vary. Hence, themanufacturing yield and cost of integrated circuits may respectivelydecrease and increase. Therefore, after ignition, the plasma should bepowered by magnetic energy, and the amplitude of capacitive electricenergy incident upon the wafer should be uniform, or approximately zero.

The Johnson Patent suggests suppressing the capacitive electric energyin the chamber with a non-magnetic conductor, known as a shield, whichis placed between the coil and the chamber. The shield may be fully orpartially coextensive with the surface of the coil. The shield hasslots, and is electrically grounded or floating. The slots suppress eddycurrents that circulate in the shield and cause undesired energydissipation. Examples of shield designs are found in the Johnson Patent.

The Johnson Patent discloses that the capacitive electric energy fromthe coil is useful for striking the plasma. Therefore, the JohnsonPatent describes a system with a mechanical shutter to vary thedimensions of the slots in the shield. The mechanical shutter is openedwhen the plasma is ignited. Because the slot dimensions are increasedwhen the shutter is opened, the capacitive electric energy coupled fromthe coil into the chamber increases. After the plasma is ignited, theshutters are closed to reduce the amount of capacitive electric energysupplied to the chamber.

The mechanical shutter disclosed in the Johnson Patent must be operatedby a motor and a control system. This technique is relatively complex,and may be prone to reliability and repeatability problems. Therefore,there is a need for a plasma reactor design that uses a less complexdesign.

SUMMARY OF THE INVENTION

The present invention solves the above-mentioned problems in the art andother problems which will be understood by those skilled in the art uponreading and understanding the present specification. The presentinvention is a plasma reactor that comprises an electromagnetic energysource coupled to a radiator through first and second variable impedancenetworks. The plasma reactor includes a chamber having a dielectricwindow that is proximate to the radiator. A shield is positioned betweenthe radiator and the dielectric window. The shield substantially coversa surface of the radiator near the dielectric window. A portion of theradiator that is not covered by the shield is proximate to a conductivewall of the chamber.

In one embodiment, the second variable impedance network is a variablecapacitor. In another embodiment, the radiator is a coil, such as aplanar coil.

Plasma reactor operation will now be described. A plasma is ignited in achamber with substantially capacitive electric energy coupled from theradiator. A variable impedance network is tuned so that the capacitiveelectric energy coupled into the chamber is diminished after ignition.The plasma is then powered with substantially magnetic energy. As aresult, the lifetime of the plasma reactor is extended. Also, thecleaning and down time of the plasma reactor is reduced.

In one embodiment, the chamber pressure is set to be less thanapproximately eighty millitorr prior to the ignition step. The chamberpressure is then set to be less than approximately thirty millitorrafter the ignition step. A second variable impedance network is tunedupon varying the chamber pressure.

It is a benefit of the present invention that it requires a lesspowerful and costly electromagnetic energy source. It is also anadvantage of the present invention that it may be used in plasmareactors for both material deposition and removal. Further features andadvantages of the present invention, as well as the structure andoperation of various embodiments of the present invention, are describedin detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements.

FIG. 1A illustrates a cross-sectional diagram of a prior art Lam TCPplasma reactor;

FIG. 1B illustrates a cross-sectional diagram of a prior art AppliedMaterials HDP plasma reactor;

FIG. 2A illustrates a schematic diagram of a plasma reactor;

FIG. 2B illustrates a schematic diagram of a first variable impedancenetwork;

FIG. 2C illustrates a schematic diagram of a second variable impedancenetwork;

FIG. 2D illustrates a cross-sectional diagram of a plasma reactor;

FIG. 3A illustrates a plot of electric potential versus radiatorposition when a relatively high electric potential exists between theexposed portion of the radiator and the nearby conducting wall; and

FIG. 3B illustrates a plot of electric potential versus radiatorposition when a relatively low electric potential exists between theexposed portion of the radiator and the nearby conducting wall.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The terms wafer andsubstrate used in the following description include any structure havingan exposed surface with which to form the integrated circuit (IC)structure of the invention. The term substrate is understood to includesemiconductor wafers. The term substrate is also used to refer tosemiconductor structures during processing, and may include other layersthat have been fabricated thereupon. Both wafer and substrate includedoped and undoped semiconductors, epitaxial semiconductor layerssupported by a base semiconductor or insulator, as well as othersemiconductor structures well known to one skilled in the art. The termconductor is understood to include semiconductors, and the terminsulator is defined to include any material that is less electricallyconductive than the materials referred to as conductors. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the claims,along with the full scope of equivalents to which such claims areentitled.

FIG. 2A illustrates a schematic diagram of one embodiment of the presentinvention, an inductively coupled plasma reactor 100. The plasma reactor100 includes a first electromagnetic energy source 102 coupled to anelectromagnetic radiator 106 through first and secondvariable-impedance-matching networks 104, 116. The electromagneticenergy source 102 emits radio frequencies of less than one hundredmegahertz. However, other embodiments of the invention are envisionedthat operate at higher frequencies. The radiator 106 may be a planar 122or cylindrical 132 coil. The first variable-impedance-matching network104 includes a series variable capacitor 202 coupled to a shunt variablecapacitor 204 and a series fixed capacitor 206, as shown in FIG. 2B. Thesecond variable-impedance-matching network 116 comprises a variablecapacitor 208, as shown in FIG. 2C. However, other electrical networktopologies may be used for the first and secondvariable-impedance-matching networks 104, 116.

The radiator 106 is positioned proximate to the dielectric window 110 ofthe plasma reactor 100. A shield 108, such as a Faraday shield, isinserted between the radiator 106 and the dielectric window 110. Theshield 108 is designed so that it substantially covers the surface ofthe radiator 106 that is near to the dielectric window 110, asillustrated in FIG. 2D. In one embodiment, the shield 108 is circularand has a diameter 170 at least one half, but less than, the diameter172 of a radiator 106 that is a planar coil 122. As a result, only arelatively small portion of the radiator 106 will be exposed through thedielectric window 110 to a conductive wall of the chamber 112. Theexposed portion of the radiator 106 is relatively close to theconductive wall of the chamber 112. Therefore, relatively littlecapacitive electric energy is required to create an electric fieldnecessary to ignite the plasma. Thus, a less powerful and less costlyelectromagnetic energy source 102 can be used in the plasma reactor 100.

A second electromagnetic energy source 118 may optionally be coupled tothe wafer 114 in the chamber 112. The second electromagnetic energysource 118 is used particularly in plasma etchers to control thedirection, and thus enhance the anisotropic nature, of the etch.

The operation of the present invention will now be described. Initially,a wafer 114, such as a semiconductor substrate, is placed into thechamber 112. Then, the electromagnetic energy source 102 is turned on.Next, the first variable-impedance-matching network 104 is tuned toenhance the energy transfer between the electromagnetic energy source102 and the radiator 106. Also, the second variable-impedance-matchingnetwork 116 is tuned to vary the voltage-standing-wave on the radiator106 so that a relatively high electric potential 302 exists between theexposed portion of the radiator 106 and the nearby conductive wall ofthe chamber 112, as shown in FIG. 3A. In this case, for example, thevariable capacitor 208 would be tuned to a relatively low capacitancevalue. The pressure of the gas in the chamber 112 is reduced to lessthan approximately 80 millitorr. Below this pressure, enough particlesare present in the plasma reactor to permit an avalanche effect to becreated by electrons accelerated by the capacitive electric energy.Thus, the capacitive electric energy coupled into the chamber 112 by theradiator 106 ignites the plasma. Plasma ignition is complete when theplasma is substantially powered by magnetic energy. For example, whenthe plasma ignition is complete, the plasma density is greater than 10¹⁰cm⁻³. After plasma ignition, the pressure in the chamber 112 is reducedto less than approximately 30 millitorr, or whatever is desired forsubsequent processing. Because the chamber pressure changes, the firstvariable impedance network 104 may be retuned to enhance the energytransfer between the electromagnetic energy source 102 and the plasma.When the plasma is nearly fully powered, the plasma remainssubstantially powered by magnetic energy and may have a density greaterthan 10 ¹¹ cm⁻³. The second variable-impedance-matching network 116 isalso tuned, suddenly or slowly, to vary the voltage-standing-wave on theradiator 106 so that the region of relatively high electric potential ofthe radiator 106 is repositioned to a portion of the radiator 106 thatis electrically isolated from the chamber 112, or unexposed, by theshield 108, as shown in FIG. 3B. After ignition, the secondvariable-impedance-matching network 116 may be tuned immediately orafter a delay. Thus, a relatively low electric potential 304 existsbetween the exposed portion of the radiator and the nearby conductingwall. As a result, the deterioration of the chamber 112 andsemiconductor process is diminished. In this case, for example, thevariable capacitor 208 would be tuned to a relatively high capacitancevalue.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This patent isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

1. A plasma reactor, comprising: an electromagnetic energy source; afirst impedance network operatively coupled to the electromagneticenergy source; a radiator operatively coupled to the electromagneticenergy source by the first impedance network; a second impedance networkserially coupled to the radiator and operatively coupled to theelectromagnetic energy source; a chamber having a window that isproximate to the radiator and a conductive wall; a shield, positionedbetween the radiator and the window, substantially covering a surface ofthe radiator near the window; a portion of the radiator that is notcovered by the shield is proximate to the conductive wall; and whereinthe first impedance network includes: a first capacitor and a second,fixed capacitor serially connected between the electromagnetic energysource and the radiator, an electrical connection between the firstcapacitor and the second capacitor, and a third capacitor having a firstplate connected at the electrical connection and having a second plateconnected to ground.
 2. The plasma reactor of claim 1, wherein theelectromagnetic energy source operates at a frequency of less than onehundred megahertz.
 3. The plasma reactor of claim 1, wherein the chambercomprises a dielectric liner.
 4. The plasma reactor of claim 1, whereinthe second impedance network includes a variable capacitor.
 5. Theplasma reactor of claim 1, wherein the chamber comprises a chamberadapted to hold a plasma that causes constituents of precursor gases inthe chamber to form a material on a substrate in the chamber.
 6. Theplasma reactor of claim 1, further comprising a plasma that removesmaterial from a substrate placeable in the chamber.
 7. The plasmareactor of claim 1, further comprising a second electromagnetic energysource coupled to a wafer in the chamber.
 8. The plasma reactor of claim1, wherein the radiator is a coil.
 9. The plasma reactor of claim 8,wherein the coil is a planar coil.
 10. The plasma reactor of claim 1,wherein the first impedance network comprises a variable impedancenetwork.
 11. The plasma reactor of claim 1, wherein the first capacitoris a variable capacitor.
 12. The plasma reactor of claim 1, wherein thethird capacitor is a variable capacitor.
 13. The plasma reactor of claim1, wherein the third capacitor is a shunt capacitor.
 14. The plasmareactor of claim 1, wherein the window comprises a dielectric window.15. A plasma reactor, comprising: a radio frequency energy source; animpedance network; a first capacitor; a coil, operatively coupled to theradio frequency energy source by the impedance network and the firstcapacitor; a chamber having a dielectric window that is proximate to thecoil and having a conductive wall; a shield, positioned between the coiland the dielectric window, substantially covering a surface of the coilnear the dielectric window; wherein a portion of the coil that is notcovered by the shield is proximate to the conductive wall of thechamber; and wherein the impedance network includes: a second capacitorserially connected to the radio frequency energy source, a thirdcapacitor connected in parallel across the second capacitor and theradio frequency energy source, and a fourth capacitor serially connectedbetween the second capacitor and the coil.
 16. The plasma reactor ofclaim 15, wherein the radio frequency energy source operates at afrequency of less than one hundred megahertz.
 17. The plasma reactor ofclaim 15, wherein the chamber comprises a dielectric liner.
 18. Theplasma reactor of claim 15, wherein the chamber comprises a chamberadapted to hold a plasma that causes constituents of precursor gases inthe chamber to form a material on a substrate in the chamber.
 19. Theplasma reactor of claim 15, further comprising a plasma that removesmaterial from a substrate placeable in the chamber.
 20. The plasmareactor of claim 15, further comprising an electromagnetic energy sourcecoupled to a substrate in the chamber.
 21. The plasma reactor of claim15, wherein the coil is a planar coil.
 22. The plasma reactor of claim15, wherein the shield comprises a Faraday shield.
 23. The plasmareactor of claim 15, wherein the impedance network is a variableimpedance network.
 24. The plasma reactor of claim 15, wherein at leastone of the first capacitor, second capacitor and the third capacitor isa variable capacitor.
 25. A plasma reactor, comprising: a chamber havinga dielectric window; a radio frequency energy source; a radiatorpositioned outside the chamber next to the dielectric window; and animpedance network coupled between the radio frequency energy source anda first end of the radiator, and including: a first capacitor and asecond capacitor serially connected between the radio frequency energysource and the radiator, and a third capacitor having a first plateconnected to the electrical connection between the first capacitor andthe second capacitor, and having a second plate connected to ground. 26.The plasma reactor of claim 25, further including a second, variableimpedance network connected between a second end of the radiator andground.
 27. The plasma reactor of claim 26, wherein the second variableimpedance network includes a variable capacitor connected between thesecond end of the radiator and ground.
 28. The plasma reactor of claim25, wherein at least one of the first capacitor and the third capacitoris a variable capacitor.
 29. The plasma reactor of claim 25, wherein thesecond capacitor is a fixed shunt capacitor.
 30. A plasma reactor,comprising: a chamber having a dielectric window; a radio frequencyenergy source; a radiator coil positioned outside the chamber next tothe dielectric window; a Faraday shield positioned between thedielectric window and the radiator coil; and a first impedance networkelectrically coupled between the radio frequency energy source and afirst location on the radiator coil, and having a first capacitor and asecond capacitor serially connected between the radio frequency energysource and the first location on the radiator, and a third capacitorhaving a first plate connected to the electrical connection between thefirst capacitor and the second capacitor, and having a second plateconnected to ground; and a second, variable impedance networkelectrically coupled to a second location on the radiator coil, andhaving a fourth capacitor connected between the second location on theradiator coil and ground.
 31. The plasma reactor of claim 30, whereinthe fourth capacitor is a variable capacitor.
 32. The plasma reactor ofclaim 30, wherein at least one of the first and third capacitors is avariable capacitor.
 33. The plasma reactor of claim 30, wherein thethird capacitor is a fixed, shunt capacitor.
 34. The plasma reactor ofclaim 30, wherein the first impedance network is a variable impedancenetwork.
 35. A plasma reactor, comprising: an electromagnetic energysource; a first impedance network operatively coupled to theelectromagnetic energy source; a radiator operatively coupled to theelectromagnetic energy source by the first impedance network; a secondimpedance network serially coupled to the radiator and operativelycoupled to the electromagnetic energy source; a chamber having a windowthat is proximate to the radiator and a conductive wall; a shield,positioned between the radiator and the window, substantially covering asurface of the radiator near the window; wherein a first impedance isset by the first impedance network and the second impedance network, thefirst impedance being adapted to enhance energy transfer from theradiator to the chamber to ignite a plasma; and wherein a secondimpedance is set by the first impedance network and the second impedancenetwork, the second impedance being adapted to sustain the plasma. 36.The plasma reactor of claim 35, wherein the second impedance is furtheradapted to reposition a relatively high electric potential to a portionof the radiator that is electrically isolated from the chamber.
 37. Theplasma reactor of claim 35, wherein the second impedance is furtheradapted to reposition a relatively high electric potential to a portionof the radiator that is unexposed by the shield.
 38. A plasma reactor,comprising: an impedance network adapted to provide a first impedanceand a second impedance; an electromagnetic energy source; a radiatoroperatively coupled to the electromagnetic energy source by theimpedance network; a chamber; wherein the first impedance is adapted toenhance energy transfer from the radiator to the chamber to ignite aplasma; and wherein the second impedance is adapted to sustain theplasma.
 39. A plasma reactor, comprising: means for igniting a plasmawith a first impedance between an electromagnetic source and a radiator;and means for sustaining the plasma with a second impedance.